Above and below the term structural adhesive refers to adhesives the precursor of which comprises a cross-linkable polymer. Depending on the nature of the cross-linkable groups the precursor may be curable to a cross-linked structural adhesive by exposing it to heat and/or actinic radiation, or it may be curable by exposure to ambient moisture. In case the precursor is heat and/or radiation curable the precursor further comprises an effective amount of a curing system. The curing system comprises one or more compounds selected from a group comprising heat-activatable compounds, and radiation-activatable curing agents.
Structural adhesives generally exhibit good adhesion properties to many surfaces and a high bond strength. They have therefore been used to replace or augment conventional joining techniques such as welding or employing mechanical fasteners such as nuts and bolts, screws, rivets etc.
It has, however, been difficult to provide a sufficient adhesion of structural adhesives to metal surfaces which are contaminated with hydrocarbon-containing material such as mineral oils, processing aids such as deep-drawing agents, lubricating agents such as dry lubes, grease, soil and the like.
It is well-known that removing hydrocarbon-containing material from surfaces can be extremely difficult. Mechanical processes such as dry wiping and/or the use of pressurized air tend to leave a thin layer of the hydrocarbon-containing material on the metal surface. Liquid cleaning compositions like that disclosed in U.S. Pat. No. 6,849,589 can be effective but may be less desirable from a processing point of view because the cleaning liquid must be collected and recycled or discarded. In addition to that a drying period is usually required after the cleaning step.
WO 2006/028,806 which discloses an encapsulated cure system for curable compositions mentions that the adhesive strength of such composition on oily metal sheets may be improved by the inclusion of liposoluble additives such as limonene, dipentene, terpene resins or oil of turpentine in an amount of 1-10 wt. % relative to the weight of the curable composition. It was found, however, that the addition of liposoluble additives to the precursor of structural adhesives does not tend to improve the adhesion of the cured structural adhesive to metal surfaces contaminated with hydrocarbon-containing soils to a practically relevant and/or desirable extent. Increasing the amount of the liposoluble additives tends to decrease overall adhesive properties.
It is therefore an object of the present invention to provide a high bonding strength assembly comprising at least one substrate having a metal surface area being bonded to a structural adhesive whereby said surface area may be contaminated with hydrocarbon-containing material. It is another object of the present invention to provide a high bonding strength assembly comprising two substrates sandwiching a structural adhesive wherein at least one substrate comprises a metal surface area bonded to the structural adhesive which may be contaminated with hydrocarbon-containing material. It is particularly preferred that such assembly exhibits a high average peel force and a cohesive failure mechanism. It is another object of the present invention to provide a method for manufacturing such assembly which comprises a step of treating the optionally contaminated designated metal surface area whereby such method is environmentally friendly, cost effective and can be integrated into the industrial process of automotive body production. It is another object of the present invention to provide a process for manufacturing said assembly where the precursor of the structural adhesive can be applied directly and without any waiting time after the surface treatment of the metal surface. It is another object of the present invention to provide an applicator device which is suitable for carrying out such process and which allows, in particular, for a contact-free treatment of the metal surface contaminated with hydrocarbon-containing material. Other objects of the present invention can be readily taken by the person skilled in the art from the following detailed specification.